1. Field of the Invention
The invention relates to optical-electrical image reproducing devices and somewhat more particularly to a method of producing insulating ring zone at orifice areas of through-holes in a plate useful in such devices.
2. Prior Art
In flat plasma picture screens, for example, such as disclosed in German Auslegechrift No. 24 12 869, corresponding to U.S. Pat. No. 3,956,667, electron flow from a plasma space to areas of luminscent material on a picture screen is regulated for each image point by an electrode matrix applied to a so-called control plate. This control plate is somewhat larger than the picture screen itself, consists of glass and is provided with through-holes corresponding to a desired pattern of image points and is coated on both sides with metallic electrode leads.
In order to improve control properties, two additional electrode levels (tetrode and pentode), which are located on a further glass plate, can be attached between the control plate and the picture screen. Both of these plates can consist of relatively thin glass layers (for example, 0.2 mm thick) or of a photoform glass layer (for example 1 mm thick) or a photoform glass layer and a thin glass layer can be combined.
With the use of photoform glass layers having through-holes therein, the hole walls must also be metallized. However, this metallization must be interrupted at a specific location for a relatively short distance in comparison to the hole depth so as to define an insulating ring zone. As a rule, such insulating ring zones should be located at the narrowest hole cross-section so that, with a photoform glass plate etched at both sides, the insulating ring lies along the center of the hole, and with single-sided etching, the insulating ring is located directly under the electrode level having the smaller hole orifice. As may be appreciated, the production of such insulating ring zones is difficult because of the fineness and multitude of through-holes.